Abstract. We interpret here the variability of levels of carbonaceous aerosols based on
a 12 yr database from 78 monitoring stations across Spain specially compiled
for this article. Data did not evidence any spatial trends of carbonaceous
aerosols across the country. Conversely, results show marked differences in
average concentrations from the cleanest, most remote sites (around
1 μg m−3 of non-mineral carbon (nmC), mostly made of organic
carbon (OC) with very little elemental carbon (EC),
around 0.1 μg m−3; OC / EC = 12–15), to the highly polluted major
cities (8–10 μg m−3 of nmC; 3–4 μg m−3 of
EC; 4–5 μg m−3 of OC; OC / EC = 1–2). Thus, urban (and very
specific industrial) pollution was found to markedly increase levels of
carbonaceous aerosols in Spain, with much lower impact of biomass burning and of biogenic emissions.
Correlations between yearly averaged OC / EC and EC concentrations adjust very
well to a potential equation (OC = 3.37 EC0.326, R2 = 0.8). A
similar equation is obtained when including average concentrations obtained
at other European sites (OC = 3.60EC0.491, R2 = 0.7).

A clear seasonal variability in OC and EC concentrations was detected. Both
OC and EC concentrations were higher during winter at the traffic and urban
sites, but OC increased during the warmer months at the rural sites. Hourly
equivalent black carbon (EBC) concentrations at urban sites accurately depict
road traffic contributions, varying with distance from road, traffic volume and
density, mixing-layer height and wind speed. Weekday urban rush-hour EBC
peaks are mimicked by concentrations of primary gaseous emissions from road
traffic, whereas a single midday peak is characteristic of remote and rural
sites. Decreasing annual trends for carbonaceous aerosols were observed
between 1999 and 2011 at a large number of stations, probably reflecting the
impact of the EURO4 and EURO5 standards in reducing the diesel PM emissions.
This has resulted in some cases in an increasing trend for
NO2 / (OC + EC) ratios as these standards have been much less effective
for the abatement of NOx exhaust emissions in passenger diesel
cars. This study concludes that EC, EBC, and especially nmC and OC + EC
are very good candidates for new air quality standards since they cover both
emission impact and health-related issues.